The use of monoclonal antibody Y1/82A in the identification of acute myeloblastic and monocytic leukemias

A newly developed monoclonal antibody (Y1/82A), which binds a cytoplasmic antigen in peripheral blood monocytes and tissue macrophages, was tested for its ability to detect monocytic differentiation in acute myeloid leukemia, i.e., to distinguish French-American-British (FAB) groups M4 and M5 from FAB groups M1, M2, M3, M6 and M7. Staining was performed by the alkaline phosphatase-antialkaline phosphatase (APAAP) immunocytochemical technic on bone marrow smears from 29 cases of acute myeloid leukemia, on 17 normal peripheral blood and/or bone marrow smears, on bone marrows from 10 cases of lymphoid leukemia, and on lymph nodes of 13 patients with lymphoma. Neoplastic cells from 11 of 11 patients with either M4 or M5 leukemia had positive results, whereas only 2 out of 18 cases of M1, M2, M3, M6, and M7 leukemia had positive results. In normal samples, only peripheral blood monocytes, bone marrow macrophages, and megakaryocytes stained. Lymphoid neoplasms were unreactive. These results suggest that monoclonal antibody Y1/82A may be a useful reagent in detecting cases of M4 and M5 acute myeloid leukemia and that it offers a valuable alternative to nonspecific esterase cytochemistry.     

A new monoclonal antibody (KB61) recognizing a novel antigen which is selectively expressed on a subpopulation of human B lymphocytes.

The present paper describes a new monoclonal antibody (KB61) raised against hairy cell leukaemia cells. Antibody KB61 recognizes a molecule of approximately 40,000 molecular weight on human B cells. It reacts with B lymphocytes in the peripheral blood, in primary lymphoid follicles, in the mantle zone of secondary follicles, in interfollicular areas and in splenic marginal zone areas. However, germinal centre lymphoid cells do not express the antigen recognized by antibody KB61. The antibody shows limited reactivity outside the lymphoid system, i.e. polymorphs, tissue macrophages endothelial cells in the hepatic sinusoids. Antibody KB61 discriminates between different types of B-cell malignancies, reacting with the neoplastic cells in hairy cell leukaemia, chronic lymphocytic leukaemia (of B-cell type), prolymphocytic leukaemia and centrocytic lymphoma, but not with acute lymphoblastic leukaemia, germinal centre-derived lymphomas (other than centrocytic), Burkitt's lymphoma and lymphoblastic lymphoma. Antibody KB61 may be of value in the study of B-cell subpopulations and in the differential diagnosis of B-cell neoplasms.     

A novel human B-lymphocyte antigen shared with lymphoid dendritic cells: characterization by monoclonal antibody.

A novel cell-surface antigen (L25) expressed on human B cells was identified using a B cell-reactive monoclonal antibody (TB1-4D5). This L25 antigen was expressed on most B-lineage cells but not other cell types including thymocytes, T cells, granulocytes and monocytes. Thus, L25 existed on the majority of normal B cells present in the blood and lymphoid tissues, on cultured cell lines derived from normal and malignant B cells, and on neoplastic cells isolated from patients with B cell-derived malignancies. Though L25 was persistently expressed on B cells until 7 days after their activation with pokeweed mitogen (PWM), neither normal nor neoplastic plasma cells expressed L25. Moreover, L25 was present on cultured as well as freshly isolated leukaemic cells with common acute lymphatic leukaemia (CALL) antigen, which have been thought to correspond to the early B-cell ontogeny. Besides pan-B cell reactivity of TB1-4D5 antibody, it apparently cross-reacted with so-called dendritic or interdigitating cells located in the thymic-dependent areas of peripheral lymphoid organs, which have been presumably ascribed to those associated with accessory-cell function. Functional studies showed that anti-L25 (TB1-4D5) antibody had inhibitory effect on induction of immunoglobulin synthesis by PWM-stimulated B cells.     

Lymphoma-like presentation of acute monocytic leukaemia

Four patients in whom a diagnosis of acute monocytic leukaemia (M5) was subsequently made presented with extramedullary disease clinically resembling lymphoma. In all patients histological sections were initially misinterpreted as showing malignant lymphoma or anaplastic carcinoma. The diagnosis of M5 leukaemia was subsequently made on the basis of morphological and cytochemical studies of peripheral blood and bone marrow. The histological diagnosis of the soft tissue lesions of M5 leukaemia (monocytic sarcoma) is difficult, although features such as abundant cytoplasm and the presence of some reniform nuclei are helpful. If there is no peripheral blood or bone marrow involvement and only fixed paraffin-embedded tissues are available, demonstration of lysozyme by an immunoperoxidase technique may confirm the diagnosis but results are not invariably positive. An early diagnosis of M5 leukaemia has therapeutic implications since the disease evolves through a progressive leukaemia phase and systemic therapy is essential.     

Antibody-dependent haemolytic activity of human leukaemic cells.

Antibody-dependent cellular cytotoxicity (ADCC) of human leukaemic blood cells against human RBC treated with IgG isoantibody was studied by the 51Cr-release method. ADCC in this particular system is a property of normal phagocytic cells of the monocytic and myeloid series while lymphocytes are inactive. Well differentiated leukaemic monocytes from patients with acute monocytic leukaemia were highly cytotoxic and engulfed opsonized RBC. Promyelocytic leukaemic cells from two patients with acute promyelocytic leukaemia were cytotoxic and phagocytic. Seven patients with low differentiated acute myeloblastic leukaemia had no cytotoxic or phagocytic blood cells. Leukaemic B cells from patients with chronic lymphocytic leukaemia or prolymphocytic leukaemia lacked cytotoxic and phagocytic properties. It is concluded that ADCC against isoantibody-treated human RBC may be a tool to distinguish between well and poorly differentiated leukaemic cells of the monocytic or myeloid series.     

Epithelial membrane antigen (EMA) or MUC1 expression in monocytes and monoblasts

AIMS: Epithelial membrane antigen (EMA) or MUC1 belongs to a heterogeneous group of heavily glycosylated proteins and is expressed in most normal and epithelial neoplastic cells. EMA is also expressed in plasma cells, anaplastic large cell lymphoma (Ki-1 antigen), malignant histiocytosis and erythroleukaemia. In 1996, Cheong et al. (Hematology 1996; 1: 223) demonstrated the positive expression of EMA in monoblasts. Since there were very few useful markers for differentiating subtypes of acute myeloid leukaemia with a monocytic component from the those without, a study was conducted to evaluate the prevalence of EMA expression and its relationship with known markers for monocytic-macrophage lineage (CD11c, CD14 and intracellular CD68) in monocytes and monoblasts. METHODS: EMA detection was performed by flow cytometry in monocytes and monoblasts. EMA expression was compared with other known markers of monocytic-macrophage lineage (CD11c, CD14 and intracellular CD68). Samples of purified monocytes were obtained from 20 healthy volunteers. Twenty-two cases of monocytic AML (M4 and M5) were studied and controls were selected from 20 cases of acute lymphoblastic leukaemia (ALL) and 18 cases of non-monocytic AML (M0, M1, M2, M3, and M7). RESULTS: EMA was shown to be expressed strongly on the surface of all purified monocytes. EMA expression was observed on blast cells in 18/22 (81.8%) cases of AML M4 and M5, but not in that of non-monocytic AML or ALL. In this study EMA monoclonal antibody has demonstrated a strong association (P<0.001) with all the other known markers of monocytic-macrophage lineage in acute leukaemia subtypes. EMA had also shown 100% specificity and 81.8% sensitivity in the diagnosis of AML M4 and M5. CONCLUSIONS: The monoclonal antibody EMA (clone E29) is a useful marker in the classification of acute myeloid leukaemia and can be used as a supplementary analysis for the diagnosis of acute leukemia with monocytic involvement.     

In vitro synthesis of some complement components (C1q, C3 and C4) by lymphoid tissues and circulating leucocytes in man.

Human lymphoid tissues and peripheral blood leucocytes and monocytes were studies with respect to the synthesis of complement components (C1q, C3 and C4) using an in vitro culture technique. All of the lymphoid tissues investigated (bone marrow, thymus, lymph node, spleen, tonsil, adenoid) synthesize complement components in different patterns. C3 was produced by all lymphoid tissues except the spleen, which was the only lymphoid tissue in which C4 production was regularly found. C1q synthesis was demonstrated in the spleen and adenoid cultures, and occasionally also in those of lymph node tissue. Lymphocytes in peripheral blood from normal individuals and in thoracic duct lymph, and also from patients suffering from chronic lymphatic leukaemia, do not synthesize any of these complement components. Peripheral blood leucocyte samples from normal individuals, containing 60 per cent lymphocytes and 40 per cent monocytes, do synthesize C3, however. Separation of the monocytes from these samples showed that it was in these cells that the synthesis of C3 occurred. Production of C3 by mononuclear phagocytes is also supported by the finding that peripheral blood leucocytes from patients suffering from acute monocytic leukaemia synthesize C3. C1q and C4 synthesis could not be demonstrated in any of the cultures of circulating leucocytes.     

Monoclonal antibody (UCHL1) that recognises normal and neoplastic T cells in routinely fixed tissues.

UCHL1 is a murine monoclonal antibody that recognises a 180-185 kD determinant on CD4 (72%) and CD8 (36%) positive T cells. This antibody is effective in formalin fixed and paraffin embedded tissues, using the immunoperoxidase method. One hundred and forty three cases of malignant lymphoma were examined. Neoplastic cells in 100% of cases of Mycosis fungoides (n = 10), 83% of cases of peripheral T cell lymphoma (n = 25), and 78% of cases of (T-ALL) T acute lymphoblastic lymphoma (n = 9) were stained by this antibody. In addition, staining was seen in 100% of cases of malignant histiocytosis of the intestine (n = 13), a condition now thought to be a T cell lymphoma. Two cases of true histiocytic lymphoma were also positive. This antibody stained neither the neoplastic cells in a wide range of B cell lymphomas (n = 62) nor Reed-Sternberg cells in 16 cases of Hodgkin's disease. UCHL1 also stained neoplastic cells in four cases of granulocytic sarcoma. A panel of normal tissues was similarly studied. Staining was seen in normal T cells and mucosal intraepithelial lymphocytes, macrophages, mature myeloid cells, and endometrial stromal granulocytes. UCHL1 is a monoclonal antibody that identifies T cells in formalin fixed paraffin embedded tissues, and should prove useful for diagnosing T cell lymphomas, especially when only formalin fixed tissue is available for diagnosis.     

Dermal monocytic sarcoma/monoblastic tumour: report of two cases of acute monocytic leukemia with initial dermal manifestations only

The clinical and pathological findings in 2 patients with acute monocytic leukemia (AMOL) presenting initially as multiple monoblastic tumours of the skin (monocytic sarcoma) were reviewed. The skin biopsies were originally interpreted as malignant lymphoma and the diagnosis of AMOL was established when overt bone marrow and/or peripheral blood involvement was detected. The time interval from initial skin biopsy to either blood or bone marrow involvement by AMOL was 2 and 18 mth. After diagnosis of extracutaneous dissemination, survival was less than 1 mth. Cytochemistry, immunohistochemistry and electron microscopy can aid in the diagnosis of a monocytic sarcoma. Generally, the most practical way to confirm the diagnosis, in everyday practice, on fixed paraffin-embedded tissues is the demonstration of alpha-1-antitrypsin (A1AT) and/or lysozyme by the immunoperoxidase technique.     

Monoclonal antibody against human macrophages/monocytes and granulocytes

A monoclonal antibody of the IgG2a isotype directed against lymphokine-activated human macrophage cell line (U937) was produced and characterized. By indirect immunofluorescence microscopy or binding assay, this antibody (MaG-1) reacted with human peripheral monocytes, peritoneal macrophages, alveolar macrophages, and peripheral granulocytes but not with lymphocytes, erythrocytes, and platelets. MaG-1 antigen was also expressed by 34% of bone-marrow cells which includes monocytic and granulocytic precursors, whereas lymphoid and erythroid precursor cells were found on MaG-1 negative population. Immunoprecipitation of 125I-labeled U937 cell extract with the Mag-1 antibody under nonreducing and reducing conditions yielded a specific band of 66 kD. Thus, this antibody defines a new human macrophage/monocyte and granulocyte-specific antigen and may be useful for studying differentiation of human macrophages/monocytes.     

Leukaemia-associated antigens in man. Detection by antibodies in maternal sera

A membrane antigen on peripheral blood lymphocytes from cases of chronic lymphatic leukaemia (CLL) is described. The antigen was detected by complement-dependent cytotoxicity using serum from a healthy pregnant woman, and appeared to be absent from the normal lymphocyte population in these patients. The serum was also cytotoxic for some acute leukaemia blast cells and for cultured Burkitt lymphoma cells; absorption studies suggested that the CLL antigen is identical to the acute leukaemic antigen.

Similar antibody activity was also found in a number of HL-A typing antisera and could be separated from the HL-A specificity by absorption.

Although these antibodies are probably directed against foetal antigens, the leukaemic antigen could not be demonstrated on foetal tissues.

     

CD68 reactivity of non-macrophage derived tumours in cytological specimens.

AIMS--To investigate the presence of the macrophage associated antigen CD68 in non-haematopoietic tumours. METHODS--Cytological specimens from non-macrophage derived tumours were stained using the alkaline phosphatase anti-alkaline phosphatase immunocytochemical method (APAAP) and three monoclonal anti-CD68 antibodies, Y1/82A, EBM11, and KP1. RESULTS--Reactivity of malignant cells with one or more of the antibodies was seen in 11 out of 40 adenocarcinomas and in one of seven poorly differentiated carcinomas; other neoplasms, including 10 cases of squamous carcinoma, three of malignant melanoma, and four of oat cell carcinoma were negative. Monoclonal antibody KP1 gave the strongest staining and reacted with the highest proportion of neoplastic cells. CONCLUSIONS--CD68 is expressed in a proportion of epithelial tumours although the labelling is usually less intense than in macrophages. Anti-CD68 antibodies should therefore be used as part of a panel in the diagnosis of poorly differentiated neoplasms in cytological material.     

Cellular distribution of human leucocyte adhesion molecule ICAM-3.

AIMS--To describe the distribution of the recently cloned human leucocyte adhesion molecule ICAM-3 in normal and neoplastic tissues and cell lines. METHODS--A panel of four monoclonal antibodies to ICAM-3 were used to stain cell lines and sections of human lymphoid tissues using the alkaline phosphatase-anti-alkaline phosphatase immunocytochemical method (APAAP). RESULTS--In peripheral blood ICAM-3 was detected on monocytes, granulocytes, and most lymphocytes. In sections of human lymphoid tissue the antigen was also found on most lymphocytes, but many of the proliferating B cells found in the germinal centres of secondary lymphoid follicles were ICAM-3 negative. ICAM-3 was also found on neoplastic white cells (in chronic lymphocytic leukaemia, hairy cell leukaemia, acute and chronic myeloid leukaemia, and multiple myeloma) with the exception of Reed-Sternberg cells in Hodgkin's disease, many of which were negative. ICAM-3 was consistently absent from cells and tissues of non-haemopoietic origin. Endothelium (which expresses ICAM-1) was negative for ICAM-3, with the exception of vessels in some neoplastic lymphoid samples which showed variable staining for ICAM-3. CONCLUSIONS--These findings suggest that ICAM-3 is essentially restricted to the haemopoietic system and is reciprocal in its expression to ICAM-1, in that it is present on resting cells and its level falls as a result of cell activation.     

Immunohistochemical characterization of Ki-M6 monoclonal antibody in Bouin-fixed,paraffin-embedded sections of normal and neoplastic human tissues

Summary A monoclonal antibody (Ki-M6) against the CD 68 antigen, which labels cells of the monocyte/macrophage system, was tested on Bouin-fixed, paraffin-embedded samples of normal, reactive and neoplastic tissues by an avidin-biotin-peroxidase complex method, with the aim of establishing its value in diagnostic pathology. In normal human tissues, Ki-M6 reactivity was confined to the so-called resident macrophages populating normal organs under physiological conditions. Moreover, restricted reactivity against cells of macrophage lineage was observed in reactive and inflammatory lesions. Granulocytes, monocyte/macrophage-related immune accessory cells, and other analysed normal tissue structures did not reveal any reactivity. Ki-M6 was strongly reactive with the cases of benign (4/4) and malignant (15/15) fibrous histiocytomas, in addition to the true histiocytic lymphomas (3/3). Cases of granular cell tumour (2/3) showed strong reactivity with Ki-M6, whereas only few immunoreactive cells, with weak staining, were seen in the other Ki-M6-positive neoplasms [neurofibroma (3/3), benign schwannoma (1/2), ganglioneuroma (1/1), malignant schwannoma (5/9), melanoma (9/28), dermatofibrosarcoma protuberans (1/1), myelomonocytic leukaemia (3/3)]. Among the epithelial malignancies tested (47 cases), Ki-M6 was positive only in renal cell carcinoma (11/14). Malignant lymphomas of the Hodgkin (56 cases) and non-Hodgkin type (67 cases) were uniformly non-reactive. From these data, Ki-M6 appears to be an excellent marker of monocyte/macrophage-related cells and appears to be a reliable indicator for fibrous histiocytomas and true histiocytic malignancies. The availability of this additional antibody capable of staining routinely processed tissue is of practical interest.     

A monoclonal antibody recognizing human Thy-1: distribution on human and non-human primate haematopoietic cells

A monoclonal antibody designated 'antibody 390' (Ab 390) with anti-human Thy-1 reactivity was prepared by the hybridoma technique from the splenocytes of BALB/c mice immunized with human fetal brain. This antibody was shown to have anti-human Thy-1 reactivity because (1) it precipitated a molecule with a molecular weight of about 24,000 daltons, (2) it had a pattern of reactivity similar to that of previously described anti-human Thy-1 antibodies and (3) purified human Thy-1 antigen specifically inhibited binding of Ab 390 to a known antigen-positive cell line. It was the intent of this study to investigate the distribution of Thy-1 on normal and malignant haematopoietic cells in humans and non-human primates. We show here that Ab 390 did not react with human peripheral blood leucocytes, bone marrow cells or splenocytes by immunofluorescence but did react with subcapsular and cortical fetal thymocytes by peroxidase-antiperoxidase immunohistology. A section of fetal spleen demonstrated staining of connective tissue and blood vessels and rare reactive lymphocytes. Adult spleen contained Thy-1-positive cells surrounding the white pulp and in the marginal zone, but single-cell suspensions of splenocytes did not react with Ab 390. Ab 390 was tested against a variety of fresh human leukaemia cells and human cell lines and was shown to react with only the acute lymphoblastic leukaemia T cell lines RPMI 8402 and HPB-MLT. Non-human primate studies revealed reactivity with a number of T cell lines from New World primates (cotton-topped and red-bellied marmosets) and peripheral blood granulocytes (owl monkey). Our studies support previous findings that suggest that human Thy-1 may be a marker for early T lymphocytes in man, and its distribution on non-human primate T cell lines suggests the same for certain species of non-human primates. Not consistent with the distribution on human cells was the demonstration of Ab 390 reactivity with owl monkey granulocytes.     

Bone marrow evaluation of monocytosis

An absolute peripheral blood (PB) monocytosis is defined as greater than 1000 monocytes/μl. The differential diagnosis of an absolute peripheral monocytosis includes a reactive monocytosis and a neoplastic process that may be associated with various haematological neoplasms, that is chronic myelomonocytic leukaemia, myeloid and lymphoid neoplasms with eosinophilia and rearrangement of PDGFRB, juvenile myelomonocytic leukaemia, chronic myeloid leukaemia, as well as acute myeloid leukaemias with a prominent monocytic component. The diagnostic approach to a PB monocytosis, including appropriate evaluation of cytomorphological features and flow cytometric analysis of the PB, associated bone marrow findings, as well as the cytogenetic and molecular findings, will be discussed.     

CD163: a specific marker of macrophages in paraffin-embedded tissue samples

Paraffin-section immunohistochemical analysis was performed using a monoclonal antibody against CD163 to evaluate the antibody's usefulness in identifying cells of monocyte/macrophage lineage in normal and neoplastic conditions. Normal human tissue samples and samples from 211 hematopoietic disorders and 115 nonhematopoietic neoplasms were examined. The distribution of KP1 and PG-M1, monoclonal antibodies to the macrophage-associated CD68 antigen, also were evaluated for comparison. CD163 immunoreactivity was observed in resident macrophages of all normal tissue samples except splenic white pulp macrophages and germinal center tingible body macrophages. Among hematopoietic disorders and nonhematopoietic neoplasms, CD163 expression was restricted largely to cases of chronic myelomonocytic leukemia, histiocytic sarcoma, sinus histiocytosis with massive lymphadenopathy, and littoral cell angioma. Acute myeloid leukemias (AMLs) with monocytic differentiation were CD163- with the exception of 1 case of acute monoblastic leukemia. Most myeloid sarcomas also were CD163-. Compared with the CD68 antibodies, CD163 demonstrated greater specificity as a marker of disorders of monocyte/macrophage origin. However, immunohistochemical evaluation of CD163 expression does not seem to be a sensitive means of determining monocytic differentiation in AMLs in paraffin sections or establishing a diagnosis of myeloid sarcoma.     

Expression of sialyl-Lewis X, an E-selectin ligand, in inflammation, immune processes, and lymphoid tissues.

The carbohydrate structure sialyl-Lewis X (SLex) can function as a ligand for E-selectin, formerly known as endothelial leukocyte adhesion molecule-1 (ELAM-1). This study was performed to analyze the expression of SLex by leukocytes and other cell types in the context of inflammatory and immune processes. Human peripheral blood cells were examined by flow cytometry using monoclonal antibody CSLEX1 directed against SLex. Cell surface SLex was found in abundance on nearly all isolated polymorphonuclear leukocytes (PMN) and monocytes, and at low levels on a substantial portion (up to 40%) of natural killer cells. This moiety was expressed also on approximately 10% of peripheral blood T cells. Immunohistochemistry was performed on various human tissues involved in inflammatory or immune processes and on secondary lymphoid tissues. In acute appendicitis, endothelial cells of postcapillary venules expressed E-selectin, and most PMN, both within vessels and extravasated, expressed SLex. A substantial number of monocytes/macrophages in inflamed appendiceal, synovial, and dermal tissues also reacted with antibody CSLEX1; however, only rare tissue macrophages in uninflamed nonlymphoid sites showed expression of SLex. These observations are consistent with the concept that SLex on circulating PMN and monocytes functions as a ligand for endothelial E-selectin in the development of inflammatory reactions. SLex-positive lymphocytes also were seen, notably, T lymphocytes in inflamed skin. An unexpected finding was that the CSLEX1 antibody also reacted with venular endothelium in certain lymphoid tissues and in inflamed appendix, but not with endothelium in normal appendix. Whether the SLex antigen identified on endothelium represents de novo expression or passive adsorption remains to be determined.     

Mast cell and lymphoreticular infiltrates in neurofibromas. Comparison with nerve sheath tumors

Cellular heterogeneity produced by non-Schwannian elements may distinguish neurofibromas from other Schwann cell neoplasma and contribute to a different tumor biology. The present study compared cell counts of mast cells, T and B lymphocytes, and macrophages in 32 neurofibromas with those in 27 schwannomas, 9 malignant nerve sheath tumors, and 17 traumatic neuromas. Immunohistochemical and histochemical analyses were performed on formalin-fixed, paraffin-embedded tissues using two monoclonal antibodies against B-lymphocyte epitopes (LN-1 and LN-2), one monoclonal antibody against T-lymphocyte epitopes (UCHL-1), one polyclonal antibody recognizing alpha 1-antichymotrypsin (ACT), a macrophage/histiocytemarker, and toluidine blue O stains. Neurofibromas contained relatively high concentrations of mast cells significantly greater than the concentrations in other neoplastic or reactive nerve sheath tumors. Most neurofibromas also displayed moderate concentrations of LN-2 immunoreactive cells, similar to the concentrations in traumatic neuromas and not statistically different from cell counts in other tumor types. Limited, variable LN-1 and UCHL-1 immunoreactive infiltrates were detected in neurofibromas and some peripheral schwannomas. Rare or moderate ACT immunoreactivity was detected in the majority of neurofibromas, in contrast with the absence, or rare appearance, of ACT immunostaining in cranial and peripheral nerve schwannomas and moderate numbers of immunoreactive cells in many malignant nerve sheath tumors. Mast cells are an important cellular marker of neurofibromas and may participate in the pathogenesis of these neoplasms.     

Neoplastic macrophages grown from human leukaemic monocytes

Leucocyte suspensions from some cases of acute myelogenous leukaemia produced large numbers of macrophages when cultured in vitro. The macrophages increased steadily in number during the first 1-2 wk of culture. The increase was partly derived from division of the macrophages themselves, partly from augmentation from supernatant cells not adherent to the surface of the culture flask. This augmentation occurred mostly in the first 24 hr of culture. The leukaemic macrophages had many features of normal macrophages as regards light microscopy, surface receptors, phagocytic activity and cytochemistry. Under the electron microscope, however, they had many abnormal characteristics, all of which have been recorded previously as features of malignant transformation. Cultures containing the abnormal macrophages and their precursor cells caused local solid tumours when injected into immune-deprived mice. The tumours were composed of rather uniform cells many of which resembled neoplastic reticulum cells. It is concluded that human leukaemic monocytes or promonocytes can develop into macrophages with ultrastructural features of malignancy. This appears to be the first demonstration of the development of neoplastic macrophages. It is suggested that some lymph node tumours of man may be composed of similar neoplastic macrophages and their precursor cells.